1. Field of the Invention
The present invention relates to a method for assembling a cam follower device, and, in particular, to a method for mounting a cam follower to a supporting member whereby no damage occurs to the outer peripheral surface of a shaft.
2. Description of the Prior Art
Conventionally, engines used to operate an automobile or the like have various configurations, but all reciprocating piston type engines, with the exception of some types of two-cycle engines, are provided with an air inlet valve and an exhaust valve which open and close in synchronism with the rotation of the crank shaft.
Various configurations exist for a valve drive mechanism for driving these air inlet and exhaust valves. An explanation will now be given with reference to the SOHC type shown in FIG. 1, as an example. An air inlet valve 4 and an exhaust valve 5 are driven in a reciprocating manner through a pair of rocker arms 3, 3 by means of one camshaft 2 rotating at one half the speed of a crankshaft 1 in the case of a four-cycle engine. The air inlet valve 4 and exhaust valve 5 are reciprocally driven through a mechanism that the ends of the rocker arms 3, 3 slide on the peripheral surfaces of a pair of cams 6, 6 which are secured to the cam shaft 2 which is rotating synchronously with the crankshaft 1.
In recent years, efforts have been made to reduce the friction between the ends of the rocker arms 3, 3 and the peripheral surfaces of the cams 6, 6 while the engine is operating, to reduce the fuel consumption during operation of the engine. For this reason, a cam follower which rotates in concert with the rotation of the cams 6, 6 has been provided at the part where the cams 6, 6 are engaged with the rocker arms 3, 3.
Specifically, as shown in FIG. 2 and FIG. 3, a pair of supporting walls 7, 7 is provided with a space therebetween at the end of the rocker arm 3 which faces the cam 6. Each end of a shaft 8 is supportingly secured to one of the respective supporting walls 7, 7. A rotatable member 10 in the form of a short cylinder is provided on the periphery of the shaft 8 through a plurality of rollers 9, 9. The outer peripheral surface of the rotatable member 10 and the outer peripheral surface of the cam 6 come into contact with each other, so that the rotatable member 10 rotates around the shaft 8 when the cam 6 rotates.
By the provision of the rotatable member 10 in this manner, the friction between the peripheral surface of the cam 6 and the end of the rocker arm 3 facing that peripheral surface is converted from sliding friction to rolling friction, reducing the fuel consumption.
For a cam follower device of this type to operate in a stable manner over a long period of time, it is necessary for the shaft 8 to be stably secured at the supporting walls 7, 7, so that the shaft 8 which supports the rotatable member 10 does not rotate with respect to the rocker arm 3. For this reason, the shaft 8 is conventionally supported at each end of one pair of supporting walls 7, 7, as shown in FIG. 3. A pair of through holes 11, 11 are formed in aligned positions in the pair of supporting walls 7, 7. The two ends of the shaft 8 are fitted into the through holes 11, 11. In addition, using a securing jig 12 as shown in FIG. 4 and FIG. 5, the two ends of the shaft 8 are secured by caulking to the inner peripheral surfaces of the through holes 11, 11, as shown in FIG. 6. This caulking secures the shaft 8 with respect to the supporting members 7, 7 formed with the through-holes 11, 11. A support rod 13 is provided to support the rocker arm 3 during the caulking operation, as shown in FIG. 6.
In the configuration outlined above, the shaft 8 must be installed between the two supporting walls 7, 7, in such a manner that the middle section of the external peripheral surface of the shaft 8 sustains no damage. The reason is that the middle section of the external peripheral surface of the shaft 8 acts as an external track for the contact of the rollers 9, 9. If this middle section of the external peripheral surface is damaged, the durability of the rolls 9, 9 is drastically curtailed.
For this reason, in the case of conventional methods for assembling a conventional cam follower device, for example, in the method disclosed in Japanese Laid Open Pat. No. Sho-63-133851, the inner diameter R of the through-holes 11, 11 formed in the supporting walls 7, 7, must be slightly larger than the outer diameter D of the shaft 8 (D&lt;R). Therefore, when the shaft 8 is inserted into the through-holes 11, 11, there is no damage to the middle section of the external peripheral surface of the shaft 8.
However, when this type of cam follower device is being assembled, the positioning of the shaft with reference to the supporting walls 7, 7 can become uncertain. For this reason, it is difficult to determine if the shaft 8 is properly secured.
Specifically, even in the state where the two ends of the shaft 8 are positioned inside the through-holes 11, 11 of the supporting walls 7, 7, it is possible for the shaft 8 to be displaced with respect to the supporting walls 7, 7 (in the lateral direction in FIG. 6). On the other hand, the ends of the shaft 8 are only slightly deformed in depth (about 0.2 to 0.3 mm) by the action of the securing jig 12, 12. For this reason, even in the case where the shaft 8 has been slightly displaced from its true position with respect to the supporting walls 7, 7, the securing of the shaft 8 is unreliable.